Recently I was under my kitchen sink and discovered a pool of water under it. Upon further investigation I noticed a moderate drip on the cold-water valve. What was odd (to me, at least) was that turning the valve on or off had no effect on the drip rate. I then discovered that the drip was actually on the line-side of the valve (As seen in the second picture). It looks like it drips where the line connects and then dribbles off the bottom.
I have never done any plumbing beyond changing a shower head. Is this something I can fix with my limited skills, or should I call a plumber? I have never shut off water to my house before and am afraid I will mess something up horribly. If I can fix it, what do I do exactly?
That is a compression valve. Use two wrenches. Hold back on the valve body to keep it from turning, and tighten the nut with the other wrench. No need to turn the house water off. Don't go overboard with the tightening. Should only take a quarter turn to stop the leak.
If you have never made a mistake, you haven't done much.